Organizing a global list of cyanobacteria and algae from soil biocrusts evidenced great geographic and taxonomic gaps

Abstract Biocrusts determine soil stability and resiliency, with a special role played by oxygenic photoautotrophic microorganisms in these communities. We evaluated temporal and geographic trends in studies focused on these microorganisms in biocrusts. Two databases were surveyed to obtain scientific articles published from 1998 to 2020 containing the terms ‘biocrusts,’ ‘algae,’ and ‘cyanobacteria.’ Although interest in biocrusts has increased recently, their ecological importance is still little explored. The scientific articles that mentioned a species list of cyanobacteria and/or algae revealed a very heterogeneous geographic distribution of research. Biocrusts have not been explored in many regions and knowledge in the tropics, where these communities showed high species richness, is limited. Geographic gaps were detected and more detailed studies are needed, mainly where biocrust communities are threatened by anthropogenic impacts. Aiming to address these knowledge gaps, we assembled a taxonomic list of all algae and cyanobacteria found in these articles, including information on their occurrence and ecology. This review is an updated global taxonomic survey of biocrusts, which importantly reveals their high species richness of oxygenic photoautotrophic microorganisms. We believe this database will be useful to future research by providing valuable taxonomic and biogeographic information regarding algae and cyanobacteria in biocrusts.


Introduction
Biological soil crusts (biocrusts) form a 'living skin' at the soil surface in many ecosystems around the world characterized by low primary productivity, including those severely limited by low water availability, nutrient scarcity, and cold temper atur es (Belna p et al. 2001 ).Biocrusts ar e often the first comm unity to colonize earl y-successional sta ges, r esulting in c hanges that facilitate the establishment of other organisms throughout ecological succession (Zhang et al. 2016, Becerra-Absalón et al. 2019 ).This ability in determine communities establishment was evaluated in many studies focused on the ecological roles that biocrusts provide in natur al envir onments, but r ecent r esearc hes ar e also e v aluating how such communities can be managed for bioremediation of degraded soils (Antoninka et al. 2020 ).
Biocrusts can be composed of man y differ ent pr oportions of v arious oxygenic photoautotr ophic gr oups (c y anobacteria, eukaryotic algae , lichens , mosses , or liverworts) (Belnap et al. 2016 ).They also support different assemblages of decomposers, as well as a faunal food web (Belnap et al. 2001 ).Terrestrial biocrusts occur in a variety of en vironments , from deserts to temperate and humid tropical climates, where they have important ecological roles because their component organisms are efficient carbon and nitrogen fixers in different environments/settings (Johnson et al. 2007, Str auss et al. 2012, Abr antes et al. 2023 ).By releasing organic compounds that ar e ric h in carbon (C) and nitrogen (N), biocrusts act as a source of nutrients that support microbial populations, whic h ar e essential for the decomposition of soil organic matter (Sepehr et al. 2019 ).Furthermor e, biocrusts ar e important for soil stabilization since they diminish wind and water erosion (Williams et al. 1995, Chamizo et al. 2017, Gao et al. 2020 ) and retain w ater, thereb y maintaining higher humidity in soils (Delgado-Baquerizo et al. 2016, Baldauf et al. 2021 ).These effects contribute for the establishment and de v elopment of the soil biota, from microbes to plants (Antoninka et al. 2020 ).
Despite the importance of the ecological services provided by biocrusts, these communities are poorly studied, although interest has increased recently.Few studies of biocrusts were published prior to the 2000s, but a series of k e y e v ents or ganized by the community of researchers dedicated to biocrusts has brought them fr om nic he to mainstr eam.First, an initial publication in 2001 (Belnap et al. 2001 ) brought a great advance in biocrust research by demonstrating that they are a global phenomenon.A hastened internationalization of the topic follo w ed, fr om a fe w r esearc h centers to the global scientific community that exists today.Another k e y e v ent was the establishment, sustenance, and growth of a triennial international biocrust meeting, the International Workshop on Biological Soil Crusts, with four editions since 2010.Even with the recently increased focus on biocrusts, and the gr eat adv ances mentioned abov e, ther e r emains a lac k of knowledge from a large-scale perspective (Bowker et al. 2018 ).
Biocrust species composition, spatial distribution patterns, and tempor al tr ends (ov er time tr ends) ar e not widel y studied nor w ell-kno wn.Nonetheless, the limited liter atur e on biocrusts r eveals that the occurrence of each species depends on se v er al factors, such as population size in a neighboring site, effectiveness of strategies for dispersion between sites and chances of finding favorable habitats when dispersing (Samolov et al. 2020 ).Part of the reason for the limited knowledge of oxygenic photoautotrophic organisms in biocrusts is that many studies have focused on c y anobacteria (curr entl y phylum Cyanobacteriota) due the special roles these prokaryotes play in soil (Mazor et al. 1996, Roncero-Ramos et al. 2019, Abrantes et al. 2023 ).Since many c y anobacteria species can fix atmospheric N in biocrusts (Zhao et al. 2010 ), they ar e commonl y consider ed to be v ery r ele v ant in the first stages of ecological succession, contributing to the subsequent establishment of other or ganisms, suc h as some algae, lichens, fungi, mosses, and vascular plants (Weber et al. 2016, Szyja et al. 2019, Cantón et al. 2020 ).Nitr ogen fixation and mec hanisms to make this nutrient available to the surr ounding envir onment (Abr antes et al. 2023 ) are relatively better known than the other ecological services biocrusts pro vide .Some aspects of N fixation have alr eady been elucidated, suc h as its high dependence on temperature , water a vailability , light intensity , and species composition in biocrusts, since different species hav e differ ent N-fixation rates (Zhao et al. 2010 ).As an ecological service, N fixation by biocrusts contributes to soil fertility in se v er al r egions, but mainl y in the most N-limited arid and semiarid en vironments , since N is dir ectl y r elated to primary pr oduction and or ganic matter decomposition (Cantón et al. 2020 ).
Other contributions of biocrusts to environment functioning, beyond N fixation, have been described.Due to the common dominance of primary producers in biocrusts (mainly species of c y anobacteriota, algae, and bry ophytes), C fixation occurs in rele v ant r ates, whic h is especiall y important in biocrusts of arid regions where oxygenic photoautotrophic microorganisms also contribute to maintaining soil moisture (Steven et al. 2012, Cantón et al. 2020 ).In addition, c y anobacteria provide soil with resistance to erosion by water and wind, as they have exopolysaccharide sheaths that bind soil particles together, favoring the stabilization of the soil surface and the formation of soil a ggr egates (Roncer o-Ramos et al. 2019, Cantón et al. 2020 ).This ecological role is speciall y observ ed in the genus Microcoleus (Belna p et al. 2001 ) and other non-nitrogen-fixing filamentous c y anobacteria, which commonl y ar e among the first species that form new biocrusts in exposed surfaces.It is interesting to note that eukaryotic algae have potential to perform all or many of the functions mentioned above in relation to soil stabilization (Hashim et al. 2020 ), with the exception of N fixation.Ho w e v er, studies r ar el y focus on these micr oorganisms, and so their roles in biocrusts remain less understood than those of c y anobacteria.
Perhaps one of the most unknown aspects of biocrusts is their taxonomic composition, especially the fraction composed of oxygenic photoautotrophic microorganisms.It is surprising that there are so few scientific articles that describe or even only mention the species that are most commonly recorded in biocrusts from different ecosystems and their geographical distribution, since this information is essential for many scientific studies on biocrusts.In contrast to the vast literature for identifying aquatic c y anobac-teria and algae, there are almost no complete taxonomic k e ys, books, or at least species lists for identifying these micr oor ganisms in biocrusts.More information needs to be generated about the taxonomic composition of biocrusts since interest is increasing and its ecological roles, including effects on ecosystem resilience and potential for ecosystem r ecov ery, could differ according to species composition (Miller et al. 2011, Samolov et al. 2020 ).Furthermore, some ecological services of biocrusts seem to be fav ored b y species diversity.Hu et al. ( 2002 ), e.g.discov er ed that soil a ggr egation is mor e effectiv e with biocrusts that contain man y species than with those comprising just a single species.More studies focusing on biocrust services and the dependence of these services on biocrust species composition are necessary to e v aluate the true role that these communities play in different ecosystems and biomes.
T hus , in this study we r e vie wed spatial and temporal patterns of studies that focused on species of free-living algae and c y anobacteria present in biocrusts.We also produced a complete list of all species of c y anobacteria and algae mentioned in these studies .T he studies focused on symbionts in lichens or other organisms were not considered here since this could generate biased results because their ecological roles and taxonomy can be v ery differ ent fr om those of micr oalgae and c y anobacteria.Furthermore, we used the geographical distribution of the records to elabor ate a ma p and to r einforce the necessity to increase efforts to better understand the biocrusts of certain regions .T he characteristics of the environments (local climate and type of substrate) wher e eac h of the most common species are found were summarized to try establish the typical habitat fr equentl y occupied by taxa.The resulting database will be a valuable source of taxonomic information for future researchers.

Methods
A data survey was carried out consisting of a detailed search for works published from 1998 to 2020 in which the topic biological crust was found together with the words algae and/or cyanobacteria.The sear ches w ere performed in Scopus, a scientific academic database, and in Google Scholar, a more popular and more accessible database.We e v aluated tempor al c hanges in the interest by this topic based on a more general survey, which was performed with the k e yw or ds 'biological crust'/'biocrust' and 'autotr ophic micr oor ganisms, ' 'Biological crust'/'Biocrust, ' and 'algae' or 'Biological crust'/'Biocrust' and 'c y anobacteria' at 1-year interv als.Searc hes with Google Scholar detected all kinds of works, including Doctoral and Masters theses, texts for scientific dissemination and scientific articles, while those with Scopus were restricted to scientific articles.We have also performed a more complete survey, including searches for more specific k e yw or ds: 'biological crust'/'biocrust ' and 'diatoms,' 'biological crust'/'biocrust' and 'bacillariophyta,' 'biological crust'/'biocrust' and 'c hlor ophyta'/'c hlor ophyte,' 'biological crust'/'biocrust' and 'c hlor ophyceae,' 'soil biofims' and 'diatoms,' 'soil biofims' and 'bacillariophyta,' 'soil biofims' and 'c hlor ophyta'/'c hlor ophyte,' and 'soil biofims' and 'c hlor ophyceae .' T his more specific survey contributed to an increase of approximately 10% in the list of research on the topic, and the complete list (general plus the most specific surveys) was used in spatial analyses and to compare the interest in different taxonomical groups.
We focused only on scientific articles, since we belie v e ther e is greater standardization of methods as they ar e peer-r e vie wed texts.Scientific articles obtained from the two databases were used to synthesize information about the species composition of  algae and c y anobacteria present in biocrusts .T his taxonomic information was organized in a table containing the following information about each study: authors, year of publication, species identification method, species present, country where the study was performed, and title ( Supplementary Material -S1 ).The table was pr epar ed using onl y studies in whic h the species of algae and c y anobacteria w er e actuall y observ ed and identified in biocrusts, and excluded those lacking a taxonomic survey or focusing on species that wer e onl y used as experimental models.Two other tables ( Supplementary Material -S2 and S3 ) were organized to present details on the ecological conditions mentioned in the studies, mainl y r elated to the r egion, local climate, and substr ates wher e the most common species wer e found.
Beyond gener al tempor al tr ends, a mor e specific e v aluation was performed from a spatial perspective, considering the geographical distribution of the studies that focused on species of algae and c y anobacteria that compose biocrusts .T his information was synthesized in a gradient world map that was pr epar ed with the pr ogr am Qgis 3.16 using the number of papers citing species by country.
Taxonomic information was e v aluated fr om four perspectiv es.The first was from a broader view, using the dataset to detect the most commonly studied group of oxygenic photoautotrophic micr oor ganisms in biocrusts, based on the number of articles that focused only on algae, only on c y anobacteria or on both groups.The second perspective used all the articles containing a species or genera list to synthesize taxonomic information obtained at the le v el of genus or species.If a genus had species identified in some studies but not in others, the information was presented as independent taxon reports for both.Two tables were prepared containing a list of names of the most commonly reported (at least in three studies) species of c y anobacteria and algae along with the countries and regions (tropical, temperate, or polar) where each were collected.It is important to highlight that these three geogr a phic r egions wer e established exclusiv el y based on latitude, without considering more refined climatic differences or social and political aspects.Further refinement of the local climate has been organized in other databases ( Supplementary Material S2 and S3 ), which is discussed elsewhere in this text.T hus , tropical r egion was consider ed the ar ea between the Tr opic of Ca pricorn and the Tropic of Cancer; temperate, between the tropics, and Polar Circles; and polar region was considered the regions within P olar Circles).T he third perspectiv e e v aluated her e was the pattern of species richness for algae and c y anobacteria betw een tropical and temperate regions, to determine whether highly diverse comm unities wer e being neglected.Comparisons between tr opical and temperate c y anobacteria and algae richness were performed by Kruskal-Walis test (Past 4.03 Software), due to data heteroscedasticity.We focused on latitudinal patterns, as we previously detected a large difference in the number of studies between temperate and tropical regions.Although this could be considered not adequate to describe the ecology of biocrusts, we use it since this latitudinal distribution of r esearc h seems to overcome the real greater importance of habitat characteristics in determining the composition, richness, and diversity of species .T hus , the F igure 3. Gradient w orld ma p based on the number of articles citing species pr esent in biocrusts by country.c har acterization of habitats was also done here as the fourth perspective, but discussed with less emphasis.For the fourth perspective , we ha ve prepared a synthesis about the substrate and local climate wher e eac h mor e common taxon was found to resume information on its major ecological c har acteristics.
Our database was built considering the taxa as cited by authors.Ho w e v er, we used two updated databases (CyanoDB 2.0, accessed in http://www.cy anodb.cz;and Algaebase, accessed in https:// www.algaebase.org/) to check if the names are currently accepted taxonomicall y.Names hav e been updated and, in cases of small spelling errors in the articles, they have also been corrected.

Results
Based on our survey, the scientific production focusing on oxygenic photoautotrophic microorganisms in biocrusts has increased, with both algae and c y anobacteria sho wing consistent incr eases fr om 1998 to 2020 for the academic Scopus database and the more accessible Google Scholar database.Two e v ents, the publication of Belnap et al. ( 2001 ) and the International Workshop in 2010, seem to have been r ele v ant in expanding interest in biocrusts, possibly contributing to the large increases in the number of articles published and to the exponential increase in interest in this topic.Even with these increases, there was proportionall y m uc h lo w er interest in algae than in c y anobacteria (Fig. 1 ).It is interesting that although the terms biocrust and biological crusts were coined and widely used in academic works, they hav e r ecentl y become mor e common and widel y found in nonacademic texts .T her e was an almost constant incr ease in interest in biocrusts from 2000 to 2020, but focus on oxygenic photoautotr ophic micr oor ganisms of this comm unity incr eased onl y mor e r ecentl y, just after 2010.
Although interest in algae and c y anobacteria of biocrusts has been increasing, detailed information at the levels of genus or species is still r ar e for these oxygenic photoautotrophic micr oor ganisms.Although ther e hav e been hundr eds of studies on biocrusts that cite algae (1187 in Google Scholar and 369 in Scopus) or c y anobacteria (1448 in Google Scholar and 734 in Scopus) (see Fig. 1 ), only 121 were found to contain a species list.These studies with more taxonomic details support the m uc h lo w er interest in algae than in c y anobacteria, since only 36 mentioned the species of algae in samples, while 85 articles listed exclusiv el y species of c y anobacteria (Fig. 2 ). Figure 2 also presents the results regarding the methods used to identify the species.Many studies (38; 31.4% of our list) provided no information about the identification method used, but we detected some interesting trends based on the remaining scientific articles .T he studies exclusiv el y focused on c y anobacteria w er e mainl y based on modern molecular methods (40 studies), with only 16 studies based exclusively in traditional morphology-based methods .T he studies focused on algae or in both taxonomic groups were more conservative in terms of the method used to identify the micr oor ganisms, with 15 of them based exclusiv el y on mor phology and onl y 6 based exclusiv el y on molecular biology (Fig. 2 ).
The 121 articles with a species list indicate spatial tendencies, with a clear geogr a phic distribution pattern.They were produced in just 23 countries (and Antarctica) among the more than 200 countries of the world (Fig. 3 ), and almost all of them were performed in temperate regions, highlighting a lack of knowledge for the tropics (Fig. 3 ; Table 1 ).Furthermore, of these 121 scientific articles, 50% resulted from studies performed in USA (29), China (20), and Spain (12).Among the few studies that were developed between the Tropic of Capricorn and the Tropic of Cancer, important contributions came from Australia (nine studies, but only six sites in the tropical region), Brazil (four) and Mexico (four).
The most fr equentl y found algal taxa (species or genera) for the 26 articles focused on algae and c y anobacteria or only on algae are listed in Table 1 , with information on phylum and countries and regions.Bracteacoccus sp. was the most commonly cited taxa, being present in nine articles; follo w ed b y Chlorella vulgaris , and Klebsormidium sp. and Stichococcus sp. in se v en; Chlorella sp., Chlorococcum sp., and Chlamydomonas sp. in six; Diplosphaera chodatii , Elliptochloris subsphaerica , and Stichococcus bacillaris in five; Chloroidium ellipsoideum , Diplosphaera sp., Myrmecia bisecta , Klebsormidium flaccidum , and Tetracystis sp. in four; and finally, Bracteacoccus minor , Klebsormidium cf.nitens , and Nannochloris sp. in three.Among these 18 most abundant taxa (species and genera), 15 ar e c hlor ophytes and thr ee ar e c har ophytes.Other taxa wer e not listed because they were present in only few studies (less than 8% of the studies).The most cited taxa were more all found in temper ate r egions, with onl y thr ee, all gr een algae (Chlor ophyta and Charophyta), occurring also in the tropical region (Table 1 ).
Table 1.List of the most fr equentl y r eported eukaryotic algae (18 species) in scientific articles found in the Scopus database, with the number of articles in which each species is cited, the countries where studies were performed and a pie chart showing the proportion of studies in temperate (blue), tropical (red), and polar (light blue) regions .T he list was ordered by r egion, fr om temper ate to tr opical, follo w ed b y frequenc y in pa pers and finall y b y alphabetic or der.  between tropical and temperate regions for the data of c y anobacteria and comparisons were not performed for microalgae, since polar and tr opical r egions wer e not sufficientl y sampled.

Algae species (Phyllum
teria were distributed in both temperate and tropical regions, with 20 of the 22 most abundant species being in both regions and only 2 species being exclusiv el y found in temper ate r egions (Table 2 ).Scarcity of data precluded comparing algae richness by region, although the unique study for the tropics listed 12 species, which was higher than the median (three species) but smaller than the mean (16.8 species) observed in temperate regions (Fig. 4 ).For c y anobacteria there w ere 22 articles for tropical regions and 88 for temper ate r egions, whic h allo w ed us to do statistical comparisons that r e v ealed no significant differ ence (Kruskal-Walis, Chi 2 = 1.79,P -value = .177)between the richness of tropical (median = 5.0; mean = 10.8) and temperate regions (median = 5.0; mean = 7.7) (Fig. 4 ).For algae, we found 26 articles for temperate regions and only two articles for tropical and for polar regions.Even with this limitation, we performed the same statistical analysis and comparisons r e v ealed no significant difference (Kruskal-Walis, Chi 2 = 0.56, P -value = .754)between the richness of tropical (median = 7.0; mean = 7.0), temperate (median = 11.0;mean = 21.8) and polar regions (median = 23.5;mean = 23.5)(Fig. 4 ).
In relation to the environment where each of the most common species was observed, almost all papers described the local climate and substrate where the biocrusts samples were taken ( Supplementary Material -S2 and S3 ).Climate was not always clearly mentioned or adjusted to an international system.An example is environments mentioned as 'extreme' by authors, without more detail about climate classification.In some cases, the authors did not mention the climate type, but it was possible to infer through the description of local conditions described in other studies carried out in the same geogr a phic coordinates.In the temper ate r egion, the surv ey detected 13 taxa in r oc ky environments and 24 in the soil, while only soils were studied in the tr opical r egion, with 18 taxa r ecorded.T hus , the epilithic biofilms (biofilms growing on rock surfaces) were only reported for temperate regions and they were exclusively focused on c y anobacteria, while studies performed in biocrusts samples from soil were more common and focused on c y anobacteria and eukaryotic algae .T he most studied environments usually coincided with taxonomical lists showing higher species richness.

Discussion
The results of the present study show increasing scientific interest in biocrusts, which is pr obabl y a consequence of recent studies exploring the important ecological services they pro vide , such as those related to water retention and fundamental roles in N and C fixation (Sepehr et al. 2019 ).Recent studies have focused on potential management of this community for environmental restoration (Rodríguez-Caballero et al. 2018b, Blankenship et al. 2019, Antoninka et al. 2020 ), which may be more attractive than more basic studies focused on taxonomic surveys.Ho w ever, taxonomic, life history and biogeogr a phic knowledge ar e still fundamental to a more specific understanding of this little understood community and would benefit r esearc hers initiating studies in a specific r egion.Unfortunatel y, the information available in scientific articles r e v eals that ther e is little or no knowledge for man y r egions and that the taxonomy of oxygenic photoautotrophic microorganisms , mainly eukaryotes , is still poorl y explor ed for biocrusts .T his lack of knowledge is also very relevant when considering that oxy-genic photoautotr ophic or ganisms of biocrusts can be threatened by anthropogenic impacts, such as trampling (Yang et al. 2020), burning (Aanderud et al. 2019 ), and climate change (Ferrenberg et al. 2015, Ste v en et al. 2015 ).
The larger number of studies focused on c y anobacteria than on eukaryotic algae in biocrusts is due to the very important ecological roles of these prokaryotic organisms, such as their abilities to fix C and N, provide soil resistance to erosion and contribute to soil moisture retention (Zhao et al. 2010, Steven et al. 2012, Cantón et al. 2020 ).These processes cause relevant changes to the soil that favor the establishment of other organisms in biocrusts and are essential to ensure subsequent successional changes and the establishment of more complex vegetation in many systems (Zhang et al. 2016 ).Ho w e v er, e v en though eukaryotic algae are not able to fix N, this is not a justification for not performing additional studies focused on these organisms in biocrusts.Algae also play extr emel y important r oles for the establishment and de v elopment of biocrusts, being fundamental to soil stability (Sepehr et al. 2019 ), and contributing to the generation of conditions that will also determine successional changes (Hu et al. 2002 ).For example, many eukaryotic algae of biocrusts have special mechanisms for growth in sites with high salinity and drought conditions.Some desiccation tolerant algae will release exopolymeric substances (EPS) that pr e v ent water loss and contribute to the formation of biofilms by a ggr egation of cells and colonies (Kumar et al. 2018 ).The strategies adopted by algae to resist stressful conditions affect ecosystem functioning.Both EPS and the abov e-mentioned a ggr egates, e.g. can contribute to soil water retention, as observed for c y anobacteria of biocrusts (Mazor et al. 1996, Adessi et al. 2018 ).Another mechanism is the production of or ganic osmol ytes, whic h is usuall y performed by halotoler ant algae (Shetty et al. 2019 ).In addition to the effects of c y anobacteria, the changes that eukaryotic algae cause to the environment have the potential to r estor e degr aded ar eas (Condon et al. 2019 ).Sommer et al. ( 2020), e.g. e v aluated the r estor ation of an ar ea degr aded by piles of potassium tailings through the development of artificial biocrusts .T hese r esearc hers found that algae contributed to reducing the erosion caused by desiccation by providing greater water availability and soil particle a ggr egation.Colonization by biofilms is gener all y the first step in species succession and will, depending on the r egion, incr ease the structural complexity of the system so that bryophytes and vascular plants are able to grow (Ciccazzo et al. 2016, Read et al. 2016, Rubio and Lázaro 2023 ).
The gr eat heter ogeneity in the geogr a phic distribution of studies focused on biocrusts may be a consequence of their geogr a phic distribution (see Rodriguez-Caballero et al. 2018a ), or due to economic differences among countries in temperate and tropical regions, r esulting in differ ent le v els of r esearc h inv estments.Temper ate countries ar e gener all y w ealthier, as sho wn b y their higher national per capita income compared to tropical countries (Gallup et al. 2000, Sachs 2001 ).The consequence is that a v ast ar ea has not been sampled or studied, and so it cannot be affirmed that this is a result of the absence of important biocrust communities.Although m uc h mor e is known about oxygenic photoautotr ophic micr oor ganisms of biocrusts of temper ate r egions, it is possible that biocrust species diversity is higher in the tropics and that important metabolic or ecological information may be being lost or neglected in this r egion.Mac hado-de-Lima et al. ( 2019) performed a study focused on c y anobacteria in biocrusts of the Brazilian Savanna and detected a diverse community, although lacking many species typically found in temperate biocrusts.According to the concept of latitudinal diversity gradient, species diversity of terrestrial ecosystems tends to increase from polar to tropical re-gions (Pianka 1966, Willig et al. 2003 ).This has been observed for many taxonomic groups and communities and may also occur with biocrusts, but there is little liter atur e av ailable to e v aluate geogr a phic patterns .T he species richness data pr esented her e show that tropical biocrusts could indeed have biodiversity levels at least similar to those of temperate biocrusts.In an interesting study, Rodriguez-Caballero et al. ( 2018a ) applied environmental niche modeling to create a global relative suitability map showing current biocrust distribution patterns .T he result suggested that biocrusts are indeed more commonly observed in temperate regions, but they also cov er v ery extensiv e tr opical ar eas in Latin America, Sahel, South and East Africa, Middle East, India, and a large portion of Australia.T hus , more studies are necessary to clarify whether the higher diversity of oxygenic photoautotrophs in biocrusts of temperate regions is a natural geographic pattern or if it is a consequence of a lack of studies in the tropics.The latter scenario is pr obabl y due to differ ent inv estments in researc h per r egion and not fr om true differ ences in the ecological services play ed b y biocrusts.Further, since the latitudinal gradient of diversity describes a general, but not absolute, pattern for all groups of species, spatial trends for biocrusts remain completely unclear.
The high concentration of studies in some countries is also reflected in species reports, with 20 of the 22 species of c y anobacteria most-commonly cited in scientific studies being mentioned in studies in USA (total of 19 species) or China (total of 13 species).These tr ends r einforce the pr obable influence of the av ailability of funding for scientific r esearc h in these countries , which ma y be introducing bias that obscures real trends present in nature.Even considering this probable bias, some taxa are certainly observed mor e fr equentl y and ar e mor e cosmopolitan on a global scale.For example, the eukaryotic algae Klebsormidium (Charophyta) and Chlorella were reported in six and four studies, respectively, with both taxa being identified in tropical and temperate biocrusts.For c y anobacteria, the higher number of studies resulted in greater frequencies, with Nostoc , Microcoleus (including M. vaginatus ), Scytonema , and Leptolyngb y a being reported in 51, 64, 31, and 25 studies, r espectiv el y, fr om tr opical and temper ate r egions.Some of these taxa are considered very common or even cosmopolitan in biocrusts, suc h as M. v aginatus , Nostoc , and Scytonema (Büdel 2003, Samolov et al. 2020). Machado-de-Lima et al. ( 2019 ) warned that many species that are commonly recorded in temperate ecosystems were not detected in their study in a tropical region, suggesting that r ele v ant differ ences may occur and should be considered by future research.
In relation to the characteristics of the environments where biocrusts are found, our data compilation shows occurrences fr om extr emel y dry to highl y humid envir onments (Castillo-Monr oy et al. 2016, Nav as Romer o et al. 2019, Hakk oum et al. 2020, Samolov et al. 2020 ).Ho w e v er, information on environmental c har acteristics and species occurr ence seems to be limited, since there are more studies in arid and semiarid climate, and almost all gener a wer e r eported for these r egions (Román et al. 2018, Williams et al. 2018, Roncero-Ramos et al. 2019, Szyja et al. 2019, Romero et al. 2020, Zhao et al. 2021 ).Although these records could generate a perception that biocrusts have more important roles in dry areas, where they possibly have a greater repr esentativ eness in the local autotrophic biomass, it is also possible that the greater number of studies in these regions are generating a very incomplete view and more studies should be developed in humid areas to try better identify their ecological functions in these en vironments .T he species list sho w ed that the most common c y anobacteria gener a ( Nostoc , Microcoleus , Sc hizothrix , and Scytonema ) and eukaryotic algae ( Bracteacoccus , Chlamydomonas , Chlorella , Diplosphaera , and Klebsormidium ) sho w ed no clear pattern of occurr ence r elated to specific envir onments and wer e r ecorded in almost all kinds of climate or growing substrate, although there is a lack of information for epilithic biofilms in the tropics.Finally, none of the most common taxa were observed to be restrict or specific to a particular environment, showing that most species or gener a ar e pr obabl y cosmopolitan.We belie v e that a high number of cosmopolitan species makes the taxonomic identification more difficult, since there are a high number of possibilities to find one among a higher number of species.Species that are endemic or typical of a region would be more easily identified as they are expected to be found in samples from this location.T hus , a revie w of liter atur e like the pr esent study is necessary to facilitate taxonomical and ecological studies.Regarding the substrate appr oac h, it is important to understand why r oc k systems ar e m uc h less studied (13 studies) than soils (42 studies), as also observed by Weber et al. ( 2022 ).Perhaps this is due to the smaller cover a ge ar ea of these systems or because there is gr eater inter est in soils due to more potential uses by humans.Although there is greater knowledge about the functions of biocrusts in soils, such as soil nutrient enrichment, mitigation of er osion, moistur e maintenance, control of water infiltration, and soil development (Beymer and Klopatek 1991, Belnap and Gillette 1997, Belnap et al. 2001, Xiao et al. 2018 ), these biological communities can also be v ery r ele v ant in r oc ky envir onments de void of soil (Abr antes et al. 2023 ).
Standardization in the methods used to identify species must also be considered, as it differed greatly between the groups of organisms studied.Cyanobacteria are mainly identified by molecular methods, while algae identification remains based on morphologically based taxonomy.Molecular biology techniques allow us to increase accuracy and speed up the species identification process (Manoylov 2014 ).If this trend continues, it is possible that studies focusing on c y anobacteria will advance at a m uc h faster pace than those focusing on eukaryotic algae, increasing the proportional deficit in knowledge about the real importance of these micr oor ganisms in biocrusts.
In summary, the survey of the liter atur e sho w ed that there is incr easing inter est in biocrusts due to the ecological services they pro vide .T hese services ar e based essentiall y on the facilitation model in successional pr ogr ess, in whic h species fr om one stage of succession modify habitat c har acteristics in a way that facilitates the establishment of species typical of more advanced stages (Connell and Slatyer 1977 ).There is great interest in this process because it could be managed for environment restoration.The interest in biocrusts should be even higher because they are under various threats, mainly related to burning events and tr ampling (Abr antes et al. 2023 ), that can diminish or eliminate their ecological r oles.Ev en though they ar e essential thr oughout the world, studies on biocrusts have a highly heterogeneous geogr a phic distribution, with knowledge of this community lacking for man y ecosystems, particularl y those of the tr opics.Although it is not very clear, these geographic gaps may be due to a latitudinal pattern of investment in research, as countries in temperate regions are generally richer than countries located in tropical regions.Ho w ever, since the relationship with GDP was not evident, ther e ar e pr obabl y other r easons suc h as scientific workfor ce, potential resear ch lines established in the field, or ecological aspects, whic h m ust be better understood to better direct efforts in studies of this community on a global le v el.Taxonomic heterogeneity was also detected for oxygenic photoautotrophic micr oor ganisms in biocrusts, with almost all studies being focused on c y anobacteria due to their ability to fix atmospheric N. Few studies have focused on eukaryotic algae, although they also play man y important r oles in soil stabilization and species succession.It is probable that this taxonomic heterogeneity influences, and is influenced by, the geogr a phic heter ogeneity of studies.Ev en with limited information, we provide here a large list of c y anobacteria and algae taxa that have been found in biocrusts ( Table S1 ) and highlight the species most commonly found in studies by region.This r epr esents a taxonomic list that will be useful for the de v elopment of ne w studies by pr oviding r esearc hers with some support on which taxa they would likely find in their samples, although new records should also be frequent due to the small number of studies in general.We expect that the data and list we assembled, based on information, i.e. dispersed in the liter atur e, to support more detailed studies focused on c y anobacteria and algae in biocrusts and contribute to diminishing gaps in scientific production on oxygenic photoautotrophic microorganisms in biocrusts.

Figure 2 .
Figure 2. Number of scientific articles focused on biocrusts that cite a species list of c y anobacteria, algae or both.